This application relates to a starter generator which has primary use on an aircraft, and wherein the starter generator is provided with an anti-windmilling feature.
Aircraft are typically provided with an auxiliary power unit, which may be mounted in the tail of the aircraft. An auxiliary power unit (APU) typically includes a gas turbine engine which is started and drives a generator utilized to provide electrical power to start the main aircraft engines. The generator also supplies electricity for various applications. The APU is typically provided with an inlet duct to supply air for use by its gas turbine engine, and an exhaust at the tail of the vehicle. The APU is typically shut down when not needed during flight of the aircraft. However, sometimes the APU may be started during flight.
During flight, ambient air may pass through the inlet duct without a door, and may drive components of the gas turbine engine. As an example, the air may drive the compressor or turbine rotors. When these rotors are driven to rotate by the air, an output shaft from the gas turbine engine leading into the generator may also be driven.
This is called “windmilling” and is undesirable for a number of reasons. First, components of the generator may rotate, but may not be provided with adequate lubricant to bearings in the APU. Also, driving the generator can generate electrical energy which can raise challenges on the overall control of the electrical grid for the aircraft. Finally, it may sometimes be necessary to start the APU while the aircraft is in flight. Windmilling makes starting the APU more challenging.